Comet 4 Control Columns

09/10/08 16:30

Control Columns

The control columns hasn’t changed from the original prototype, in 1949, to the Nimrod MR-2. Also the Sud-Aviation Caravelle, which used a modified Comet nose. The pilot and co-pilot columns are mirror-images, except that only the pilot’s column has the nosewheel steering wheel and the stall warning stick-shaker.

The buttons on the yolk are the auto-pilot kill switch on one side, and a rotating selector switch for the radio transmitter on the other. Within the column housing, chains carried the drive from sprockets on the back of the yolk and nose steering wheel to the floor, where the drive was converted to pulleys and cables. Cables actuated the powered hydraulic servos.

The stick-shaker was essentially a motor spinning an off-center weight, like a mobile phone vibrator, but much bigger. It was triggered by stall sensors under the leading edge of the wing. It made a horrible noise; a clear early-warning to the pilot. Stick shakers were installed on the Comet 1 after two crashes: G-ALYZ, in Rome, on 26 October 1952, and CF-CUN, at Karachi, on 2 March 1953. The need came during the transition between propellor and jet propulsion. Tractor propellors assist take-off by forcing air over and under the wing, increasing lift. Jets have to rely purely on air-speed. Early jets, military and civil, lacked the advanced slats and flaps of modern jets, or the thrust of modern engines to help the pilot out of trouble. In the 1950s, it was very easy to stall by lifting the nose too early or too rapidly.

Inquiries concluded that the crashes were pilot error. In parallel with Comet development, de Havilland had continued research using the experimental prototype, DH-108, and especially low-speed work, investigating airflow on the leading edge of the wing. Chief Test Pilot, John Cunningham, who had been in charge of testing the DH-108, simulated what the pilots had done in Rome and Karachi. The cure was three-fold: slight changes to the leading edge wing profile, plus new spoilers and fences, better education for pilots, and this stick shaker as the last resort.